1. Field of the Invention
The present invention relates to methods and apparatus for interconnecting a screwed oilfield wellhead and downstream equipment. More particularly, the present invention relates to improves techniques for more reliably connecting and easily disconnecting a screwed oilfield wellhead with a flanged blowout preventer.
2. Description of the Relevant Art
There are two popular types of casing heads used in oilfield operations. An independent screwed casing head is classified by the American Petroleum Institute (API) in a publication referred to as Specification 6A. This type of casing head utilizes a threaded top-end connector with a cap or "top nut" that closes off the annulus between the surface casing and the production casing string. The casing head is typically threaded onto the top of the surface casing with the production casing rising above the casing head top nut or top connector. A tubing head is then typically threaded onto the top of the production casing string. Since the tubing head body and casing head bodies are not in contact with each other, they are classified as "independent." Also, the independent screwed casing head is not dependent upon the tubing head to form a completely pressure-sealed unit.
The second type of commonly used casing head is a flanged casing head, which generally meets the standards of API Specification 6A or may have dimensionally compatible flanges meeting the requirements of the American National Standards Institute (ANSI). The flanged casing head permits direct mating to a tubing head having the same size lower flange, rather than to a lower thread connection. The production casing is typically cut off a few inches above the top flange of the casing head, and the flanged tubing head has a lower recess or socket within a rubber seal that slides down over the protruding casing until both flanges mate. The flanged casing head is dependent upon the flange tubing head to achieve a complete seal between the surface casing and the production casing. Since the tubing head slides freely over the top of the production casing until it contacts the top flange of the casing head, the weight of the tubing is supported only by the casing head body and not by the production casing hanger. This feature reduces the chances of overloading the production casing hanger mechanism when utilizing a flanged casing head.
Flanged casing heads were initially developed for high pressure or deep wells. Flanged casing heads typically are designed for pressures in excess of 10,000 psi and for applications involving more than 5,000 meters well depth. Independent screwed casing heads, on the other hand, typically have a maximum pressure capability of only several thousand psi and typically are used on wells of less than 3,000 meters. Flanged casing heads are typically massive in size and weight compared to independent screwed casing heads due to the more stringent design of pressure and depth conditions. Accordingly, flanged casing heads are typically much more expensive than independent screwed casing heads.
While independent screwed casing heads are satisfactory for many applications, it has become increasingly more common for over a decade to utilize blowout preventers (BOPs) as mandatory equipment even when drilling and completing shallow, low pressure wells. Accordingly, most BOPs in use today have a massive API bottom flange to mate with a flanged casing head. An adapter flange, also known as a drilling flange, may be used to temporarily mate the top thread of an independent screwed casing head and therefore provide the connection between an independent screwed casing head and a flanged BOP. Since the production casing string has to be run through the BOP after drilling, the casing head is installed below the BOP, so that the casing hanger mechanism can be safely set.
Currently available drilling flanges can convert an independent screwed casing head to one that accommodates a flanged BOP. Drilling flanges are, however, unfortunately vulnerable to a phenomenon known as "thread galling," wherein the female thread in the drilling flange becomes seized to the male thread on the top end of the connection of the casing head body. When thread galling occurs, the casing head and drilling flange must be removed and replaced with a new casing head body prior to running production casing downhole. The annulus between the production casing and the surface casing thus usually cannot readily be closed off with the drilling flange left permanently in place, since the annular seal cannot be retained unless the top nut is installed. Most attempts at removing the seized drilling flange from the casing head body result in the original casing head body becoming ruined, and often the drilling flange is so badly damaged that it also has to be discarded. At the very least, the damaged flange will require considerable reconditioning at substantial expense, which is a significant drawback to the continued use of the less expensive independent screwed casing head.
In an attempt to eliminate the above-described galling problem, some manufacturers have employed a "pack-off plate" or "compression flange" with an API flanged casing head. This pack-off plate closes the annulus between the independent screwed casing head and the production casing, and thus performs the function of the top nut on the independent screwed casing head. When a pack-off plate is used, the production casing is thus permitted to rise above the casing head to allow an independent screwed tubing head to be threaded thereon, rather than using a much more expensive API flanged tubing head. Due to the relatively massive size of the standard API flange on the casing head, the diameter of the pack-off plate is relatively large, so that this procedure unfortunately is generally not economical, particularly when compared to the lower cost of the original independent screwed casing head. The continued utilization of the significantly less expensive independent screwed casing head rather than an API ranged casing head would result in a considerable equipment budget savings recognized by an oil company.
One attempt to resolve the problem described above with respect to independent screwed casing heads was developed by J. M. Huber Corporation and may be referred to as the "bolt-on flange assembly." The bolt-on flange assembly was installed on an independent screwed casing head for interconnecting the casing head to a ranged BOP. This assembly primarily consisted of a threaded adapter and a flange vertically above the adapter and supported on both the screwed casing head and the adapter. Once installed on the casing head, the bolt-on flange could remain a permanent part of the wellhead assembly. The flange could thus be used for both interconnection with a BOP and for subsequent interconnection with a flange or gland once the BOP was removed. After a drilling operation is complete, the flange also could be removed from the threaded adapter and replaced with a relatively small and inexpensive packoff flange or gland, which could then be bolted to the adapter so that the flange could be reused on another well. Since the flange is sealed directly to the wellhead, only a single flange/wellhead seal is required. This design recognized the difficulty of reliably breaking the threaded flange/wellhead connection after drilling, and obviated the problem by leaving the threaded adapter in place while reusing the more massive and expensive flange that was bolted to, and positioned vertically above, the adapter. The bolt-on flange assembly thus envisioned that the flange would mate with a BOP and that the flange could be disconnected from the threaded adapter and removed with the BOP to a new well when drilling is complete. The threaded adapter could remain in place for bolted interconnection with a much smaller flange that retains the annulus seal inside the wellhead.
While the bolt-on flange assembly diminishes the problem associated with the threads of an independent screwed casing head seizing to the point that the casing head had to be destroyed, this technique also does not fully satisfy the needs of many customers. Further improvements are required so that the independent screwed casing head may be readily adapted for attachment to a ranged BOP. Substantial equipment savings will be realized and drilling downtime decreased for many oil companies that are drilling relatively shallow, low pressure wells if such improvements are developed.
The disadvantages of the prior art are overcome by the present invention. Improved techniques and equipment are hereinafter disclosed for economically interconnecting an independent screwed casing head with a ranged BOP. The improved equipment and techniques of the present invention overcome many of the problems associated with the prior art drilling flange assemblies, and persons skilled in the art will appreciate that the present invention solves or substantially eliminates many of the problems discussed above.